Update wxMediaCtrl for GStreamer 1.0

Description

With GNOME 3.0 (april 2011), the GNOME Project decided to discontinue GConf.
It won't be maintained anymore: bugs and security holes will not be fixed.
Thus, wxWidgets shouldn't depend on GConf any longer.
Please, migrate to GSettings.
GStettings is the official GNOME replacement for GConf, and the GNOME project
is porting all of its applications to it:​https://live.gnome.org/GnomeGoals/GSettingsMigration

Summary
changed from Migrate from GConf to GSettings to Use GSettings instead of GConf in wxMediaCtrl and wxLaunchDefaultBrowser()

Thanks, gstreamer indeed depends on gconf. And there is an invocation of "gconftool-2" in source:wxWidgets/trunk/src/unix/utilsx11.cpp​ too. The latter shouldn't be difficult to fix (although I couldn't find what was the equivalent of gconftool with GSettings yet) but we probably need to wait for a next version of gstreamer to fix the former.

Summary
changed from Use GSettings instead of GConf in wxMediaCtrl and wxLaunchDefaultBrowser() to Update wxMediaCtrl for GStreamer 1.0

The use of gconftool-2 in wxDoLaunchDefaultBrowser() is to support older systems, should not be removed, and does not need to be updated to use GSettings. It will not be run on newer systems, because the earlier call to xdg-open will have succeeded.

wxMediaCtrl uses GStreamer, so any system using GStreamer 0.10 or earlier will depend on GConf. Patches for support of GStreamer 1.0 in wxMediaCtrl would be welcome.

wxGTK should simply use gtk_show_uri if gtk ver >=2.14. Though I suppose you guys still insist on supporting older, so that xdg-open business is for older than that still, and wxX11/wxUniv.

Is there any ongoing work on gstreamer 1.0 support somewhere? This 0.10 dep is one blocker for removing gst0.10 from Gentoo and I might have to step up here. And given how long 0.10 has been completely unmaintained upstream and how most distros want to get rid of it rather sooner than later, I am adjusting priority to higher (not sure anyone tracks that though).